1. Field of the Invention
The invention relates to a binder composition and membrane electrode assemblies employing the same, and more particularly to a binder composition for high temperature conductivity and membrane electrode assemblies employing the same.
2. Description of the Related Art
Fuel cells are well known and are commonly used to produce electrical energy by means of electrochemical reactions. Compared to conventional power generation apparatuses, fuel cells have advantages of causing less pollution, generating less noise, increased energy density and higher energy conversion efficiency. Fuel cells can be used in portable electronic products, home-use or plant-use power generation systems, transportation vehicles, military equipment, space industry application, large-sized power generation systems, etc.
For example, in the case of a proton exchange membrane fuel cell (PEMFC), hydrogen is supplied to an anode and an oxidation reaction occurs in the presence of an anode catalyst layer, thus protons and electrons are generated. The protons reach the cathode through the proton exchange membrane. Meanwhile, in the cathode, electrons from the anode via the external circuit are reduced to oxygen supplied to the cathode and protons by reduction, producing water.
FIG. 1A shows an exploded view of conventional fuel cell 10 with a membrane electrode assembly, and FIG. 1B shows a cross-section view of FIG. 1A. As shown in FIGS. 1A and 1B, the conventional fuel cell 10 can comprise a membrane electrode assembly 12 comprising a catalytic anode film 121, a proton exchange membrane 122, and a catalytic cathode film 123, wherein a binder composition 124 can be used to combine the catalytic anode film 121 and the proton exchange membrane 122, and/or the catalytic cathode film 123 and the proton exchange membrane 122. The conventional fuel cell 10 further comprises a bipolar plate 13 and two end electrode plates 11 for connection, wherein the bipolar plate 13 and the end electrode plates 11 comprises gas passages 111 and 131 for conducting hydrogen and oxygen into the membrane electrode assembly 12.
In general, conventional binder compositions employed by membrane electrode assemblies comprise NAFION (manufactured by E. I. Du Pont de Nemours & Co.) as main component. The NAFION has adequate physical properties, chemical properties, and proton conductivity, but some deficiencies exist when used as a main component of binder compositions. NAFION refers to a sulfonated tetrafluorethylene copolymer.
To begin, the NAFION-based binder composition exhibits swelling deformation in the present of aqueous solvent, resulting in difficulties for precision control during coating processes.
Further, under relatively high temperatures (>80° C.), NAFION is apt to softening and may be further induced to change phases, resulting in reduction of proton mobility efficiency. Additionally, under relatively low temperatures (<80° C.), the NAFION exhibits inferior CO tolerance, resulting in reduction of catalyst efficiency and decrease of the operating lifespan of fuel cells.
Moreover, for NAFION-based binder compositions, required water management is difficult to control. Inefficient water management may lead to an anode becoming prone to drying and the cathode to flooding, resulting in oxygen not being able to contact the surface of the catalyst, thus limiting proton transport.
Additionally, when operating in a temperature of more than 100° C., NAFION-based binder compositions exhibit inferior proton mobility, thus, structural deterioration may occur due to the poor water retention coefficient thereof.
Accordingly, a novel binder composition for membrane electrode assemblies to replace conventional NAFION-based binder compositions is required.